1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device having high aperture ratio, wide viewing angle and high brightness.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The long thin shapes of the liquid crystal molecules can be aligned to have an orientation in a specific direction. The alignment direction of the liquid crystal molecules can be controlled by an applied electric field. In other words, as an applied electric field changes, the alignment of the liquid crystal molecules also changes. Due to the optical anisotropy of the liquid crystal molecules, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an electric field applied to a group of liquid crystal molecules in respective pixels, a desired image can be produced by diffracting light.
There are many types liquid crystal displays (LCDs). One type of LCD is an active matrix LCD (AM-LCD) that has a matrix of pixels. Each of the pixels in an AM-LCD has a thin film transistor (TFT) and pixel electrode. AM-LCDs are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
A related art LCD includes a color filter substrate (upper substrate) having a common electrode, an array substrate (lower substrate) having a pixel electrode, and a liquid crystal layer interposed between the color filter substrate and the array substrate. In the related art LCD, the liquid crystal layer is driven by a vertical electric field between the pixel electrode and the common electrode. Accordingly, the related art LCD has increased transmittance and aperture ratio. The related art LCD, however, has a narrow viewing angle because it is driven by the vertical electric field. To solve the above problems, various types of LCDs having a wide viewing angle such as an in-plane switching (IPS) mode LCD device have been suggested.
FIG. 1 is a schematic cross-sectional view of an in-plane mode liquid crystal display device according to the related art. In FIG. 1, an in-plane switching (IPS) mode liquid crystal display (LCD) device includes first and second substrates 50 and 30 facing and spaced apart from each other, and a liquid crystal layer 90 interposed therebetween. The first substrate 50 has a plurality of pixel regions “P1” and “P2.” A thin film transistor (TFT) “T,” a common electrode 58 and a pixel electrode 72 are formed on the first substrate 50 in each pixel region “P1” and “P2.” The TFT “T” includes a gate electrode 52, a semiconductor layer 62 over the gate electrode 52, a source electrode 64 and a drain electrode 66 spaced apart from the source electrode 64. A gate insulating layer 60 is interposed between the gate electrode 52 and the semiconductor layer 62. The common electrode 58 and the pixel electrode 72 are parallel to and spaced apart from each other.
The common electrode 58 may be formed of the same material and the same layer as the gate electrode 52, and the pixel electrode 72 may be formed of the same material and the same layer as the source and drain electrodes 64 and 66. In addition, the pixel electrode 72 may be formed of a transparent material to increase aperture ratio. Even though not shown in FIG. 1, a gate line, a data line crossing the gate line and a common line supplying a common voltage to the common electrode 58 may be formed on the first substrate 50.
A black matrix 32 corresponding to the gate line, the data line and the TFT “T” is formed on the second substrate 50. A color filter layer 34 including sub-color filters 34a and 34b is formed on the black matrix 32. Each sub-color filter 34a and 34b corresponds to the pixel region “P1” and “P2.” A liquid crystal layer 90 is driven by a lateral electric field 95 between the common electrode 58 and the pixel electrode 72.
FIG. 2 is a schematic plan view of an array substrate for an in-plane switching mode liquid crystal display device according to the related art. In FIG. 2, a gate line 54 is formed on a substrate 50 and a data line 68 crosses the gate line 54 to define a pixel region “P.” In addition, a common line 56 parallel to the gate line 54 crosses the pixel region “P.” A thin film transistor (TFT) “T” including a gate electrode 52, a semiconductor layer 62, a source electrode 64 and a drain electrode 66 is connected to the gate line 54 and the data line 68. The gate electrode 52 and the source electrode 64 are connected to the gate line and the data line 68, respectively. Common electrodes 58 are formed in the pixel region “P.” The common electrodes 58 perpendicularly extend from the common line 56 and are parallel to each other. Pixel electrodes 72 alternate with and are parallel to the common electrodes 58.
FIG. 3A is a schematic view showing an OFF state of a liquid crystal layer of an in-plane switching mode liquid crystal display device according to the related art and FIG. 3B is a schematic view showing an ON state of liquid crystal molecules of an in-plane switching mode liquid crystal display device according to the related art.
In FIG. 3A, a liquid crystal molecule 90a keeps an initial state when a voltage is not applied to a common electrode 58 and a pixel electrode 72. As shown in FIG. 3B, when a voltage is applied to the common electrode 58 and the pixel electrode 72, a lateral electric field 95 is generated between the common electrode 58 and the pixel electrode 72. The liquid crystal molecule 90a rotates according to the lateral electric field 95 and is re-arranged to have an angle with respect to the lateral electric field 95. When the liquid crystal molecule 90a makes an angle of 45° with respect to the lateral electric field 95, transmittance of the liquid crystal layer is maximized. However, if a higher voltage is applied, the liquid crystal molecule 90a rotates and makes an angle less than 45° with respect to the lateral electric field 95. As a result, transmittance is reduced again.
FIG. 4 is a V-T curve showing transmittance property according to voltage of an in-plane switching mode liquid crystal display device according to the related art. As shown in FIG. 4, assuming that a maximum applied voltage is 10V, transmittance has the maximum value at 6V. As the applied voltage increases over 6V, a transmittance curve declines. When a voltage over 6V is applied, a liquid crystal molecule is re-arranged nearly parallel to a lateral electric field. Accordingly, the liquid crystal molecule makes an angle less than 45° with respect to the lateral electric field and transmittance is reduced.
FIG. 5 is a graph showing a viewing angle property of an in-plane switching mode liquid crystal display device according to the related art. As shown in FIG. 5, viewing angle property is not symmetrical according to viewpoint such as up-and-down and right-and-left of a liquid crystal panel. Accordingly, a stable wide viewing angle is not obtained and a color shift, such as yellow shift and blue shift, severely occurs. These disadvantages deteriorate display quality of an IPS mode LCD device.